Plastics are typically organic polymers of high molecular mass, but often contain other substances as well. The use of plastic expanded rapidly through the 20th century as the quality of plastics improved. Plastics are lightweight, flexible, and sturdy and can be used as a replacement for wood, metal and glass. Plastics, however, also have negative aspects. Toxic chemicals, such as benzene and dioxin, may be released into surrounding communities during the manufacture of certain types of plastic, and some types of plastics leach chemicals as they are being used. In addition, a plastic bottle tossed into a landfill may take hundreds of years to break down. Plastic bags that litter the landscape may harm animals that try to eat them, and may harm aquatic life when deposited into bodies of water.
Concerns over the environmental impact, health issues, and the rising price and supply of petroleum, have encouraged the use and development of bioplastics synthesized from corn, soy, sugar cane, and other crops. Bioplastics are now being used in deli and food containers, cups, bottles, plates and have also been used for automotive parts. Most of the bioplastic that is now being produced is polymerized lactic acid (PLA). PLA production releases fewer toxic substances than making petroleum plastic and uses less energy. It has been estimated that about two-thirds less greenhouse gas is produced during production. And PLA plastic can be composted, incinerated or recycled. PLA biodegrades relatively quickly under the right conditions, and can also repeatedly be recycled into more of the same product.
In a waste stream however, PLA plastics cannot be recycled with the bulk of petroleum-based plastic, which are primarily polyethylene terephthalate (PET) and high density polyethylene (HDPE) plastics. Even very small amounts of PLA impurity in PET or HDPE produced from recycling can lead to severe deterioration in the plastic properties. PLA plastics are similar in appearance to other plastics, and since PLA and PET bottles are similar in many other aspects such as density and transparency, the near infra-red (NIR) based sorting techniques used by a large majority of recycling processors are of only minimal utility. The inclusion of PLA bottles is also considered to take away value in the PET recycle stream by creating problems with sorting efficiency, accuracy, and potential yield loss. Better systems for identifying and sorting of plastics are therefore needed.